In an engine system, especially in a compression-ignition engine, such as a diesel engine, injectors are used in engine cylinders and exhaust gas processing systems. In both applications, the main functions of the injector include controlling a flow rate of a working fluid and atomizing the working fluid (the injectors are also called atomizing injectors).
In applications of in-cylinder fuel injections, a variety of methods can be used for controlling flow rate. Among them, a pre-metering method, in which a fluid is metered and then injected, and a common-rail method, in which a flow rate is controlled by adjusting an opening time of an injector in a repeating cycle, are commonly used. Fuel atomization in these applications is normally achieved at high injection pressure.
In exhaust gas processing systems, however, injection pressure is limited. For example, in a DPF (Diesel Particulate Filter) system, especially in a DPF system of an engine with a common rail fuel system, to regenerate the DPF, an external doser can be used for delivering fuel into a combustion device, which normally includes a DOC or a fuel burner. In the external doser, typically a lifting pump in the engine fuel system, which is primarily used for delivering low pressure diesel fuel to high pressure pumps, is employed for providing fuel to an injector, through which fuel delivery rate is controlled. The lifting pressure is much lower than the in-cylinder injection pressure. In a SCR (Selective Catalytic Reduction) system, DEF (Diesel Exhaust Fluid), which is a 32.5% wt urea solution, needs to be delivered to exhaust gas to reduce NOx therein. Limited to penetration distance, cost, and device size, normally only low pressure pumps (lower than 10 bars) are used in DEF delivery.
Though injection pressure is low, exhaust gas processing systems are sensitive to working fluid atomization. In a DPF system, poor atomization causes delay in fuel oxidation in catalyst and fuel carbonization (coking), which may block doser nozzles and deteriorate temperature control performance, while in a SCR system, large droplet of DEF lowers deNOx efficiency and increases chances of urea crystallization, which may block injectors, catalysts, and even exhaust passages.
To obtain good atomization at low injection pressure, a variety of technologies can be used. One of the most commonly used technologies is using compressed air to assist working fluid injection. In this technology, the working fluid metered by a metering pump or a metering injector is mixed with compressed air, and the result mixture is then delivered to exhaust gas. However, in the air-assisted technology, compressed air also goes into exhaust gas with the working fluid. The compressed air lowers exhaust temperature, which is critical to reactions in the exhaust gas processing system, resulting in evaporation issues, poor thermolysis, and higher energy cost, and evaporates working fluid in a mixing chamber in which the working fluid mixes with the compressed air, causing deposit and crystallization therein. Additionally, changes in compressed air pressure also vary pressure downstream from a metering injector when it is used for controlling the DEF flow rate, introducing control errors.
Another technology is using an atomization means together with an injector, as disclosed in the U.S. Pat. Nos. 8,047,452, and 6,279,603, in which an atomization device is used for creating small DEF droplets. In this technology, the working fluid has to cycle through the injector downstream from a possible pressure sensor, sensing values obtained from which are used for compensating flow rate control. Cycling the working fluid through the injector carries heat away therefrom, however, heating working fluid may cause fluid quality issues, since some working fluid deteriorates at high temperature, for example, deterioration of DEF starts to accelerate at a temperature higher than 50° C., while working fluid cycling downstream from the pressure sensor causes variations in pressure difference across the injector nozzle undetected, resulting in errors in flow rate control.
Additionally, after a fluid delivery process completes, or a long time without energizing the injector, working fluid residue in the injector may be solidified, blocking the working fluid from flowing through. The solidification process is shorted when the temperature of the working environment, in which the injector is exposed, is high.
To solve the problems mentioned above, a primary object of the present invention is to provide a working fluid injector with which a good atomization can be achieved at low injection pressure without using compressed air.
A further objective of the present invention is to provide a working fluid injector pressure drop across which can be detected accurately through a pressure sensor upstream, thereby accurate flow rate control can be obtained.
Another objective of the present invention is to provide a working fluid injector that is able to create self-spinning of working fluid droplets to decrease spray angle and penetration distance, and improve atomization at low injection pressure.
Yet another objective of the present invention is to provide a purging apparatus that is able to empty an injector when the risk of working fluid solidification is high.
Yet another objective of the present invention is to provide a control method for a working fluid injector to avoid solidification of the working fluid in a working environment with high temperature.